Exhaust gas produced by on-road vehicles in the United States currently contributes about a third of the country's smog-producing air pollution. Efforts to reduce smog include the use of more fuel efficient engines, such as diesel engines compared to gasoline engines, and improved exhaust gas treatment systems.
The largest portions of most combustion exhaust gases contain nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2), but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). One of the most burdensome components of vehicular exhaust gas is NOX, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). The production of NOX is particularly problematic for lean burn engines, such as diesel engines. To mitigate the environmental impact of NOX in exhaust gas, it is desirable to eliminate these undesirable components, preferably by a process that does not generate other noxious or toxic substances.
The exhaust gas of diesel engines tends to have more soot compared to gasoline engines. Soot emissions can be remedied by passing the soot-containing exhaust gas through a particulate filter. However, the accumulation of soot particles on the filter can cause an undesirable increase in the back pressure of the exhaust system during operation, thereby decreasing efficiency. To regenerate the filter, the accumulated carbon-based soot must be removed from the filter, for example by periodically combusting the soot by passive or active oxidation at high temperatures.
For lean burn exhaust gas, such as diesel exhaust gas, reducing reactions are generally difficult to achieve. However, one method for converting NOX in a diesel exhaust gas into more benign substances is commonly referred to as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOX, in the presence of a catalyst and with the aid of a reducing agent, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to contacting the catalyst. The reductant is absorbed onto a catalyst and the NOX reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for a stoichiometric reaction using ammonia for an SCR process is:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Original catalysts utilized in the selective catalytic reduction of NOX using NH3 as the reducing agent included metals, such as platinum or platinum group metals. Current SCR catalyst technology and research is focused on vanadium catalysts supported on titanium and tungsten (V—Ti—W) and has exhibited some success.
Since SCR catalysts generally serve as heterogeneous catalysts (i.e., solid catalyst in contact with a gas and/or liquid reactant), the catalysts are usually supported by a substrate. Preferred substrates for use in mobile applications include flow-through monoliths having a so-called honeycomb geometry that comprises multiple adjacent, parallel channels that are open on both ends and generally extend from the inlet face to the outlet face of the substrate. Each channel typically has a square, round, hexagonal, or triangular cross-sectional. Catalytic material is applied to the substrate typically as a washcoat that can be embodied on and/or in the walls of the substrate.
Many catalysts incorporating vanadates, relative to certain other SCR catalysts, do not efficiently convert NOx at operating temperatures below 300° C. Many known catalysts incorporating vanadates also lose activity upon prolonged exposure to temperatures above 600° C., which is the operating temperature for filter regeneration. Therefore, there is a need to develop an improved exhaust gas treatment system which can utilize a vanadate.